This commit alters views for the following models to use the markdown cache if
present:

* AbuseReport
* Appearance
* ApplicationSetting
* BroadcastMessage
* Group
* Issue
* Label
* MergeRequest
* Milestone
* Project

At the same time, calls to `escape_once` have been moved into the `single_line`
Banzai pipeline, so they can't be missed out by accident and the work is done
at save, rather than render, time.

Element with the content being copied cant be hidden so instead i\'ve moved it off the screen

On markdown snippets this adds a hidden div with the raw markdown content so that it can be copied

Closes #13882

I decided to create a fork of rouge as rouge lacks a HTML formatter with
the required options such as wrapping a line with <span> tags.
Furthermore I was not really convinced about the clarity of rouge's
source code.

Rugments 1.0.0beta3 for now only includes some basic linting and a new
HTML formatter. Everything else should behave the same.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>